Carcinogenesis by hepatic peroxisome proliferators: evaluation of the risk of hypolipidemic drugs and industrial plasticizers to humans

Positive feedback between ethanolamine-specific phospholipid base exchange and cytochrome P450 activities in rat liver microsomes. The effect of clofibric acid

The results of the present investigation relate the effects of the nutritional state and administration of clofibric acid (CLA), a hypolipidaemic drug and peroxisomal proliferator, on phosphatidylethanolamine (PE) synthesis in rat liver and fatty acid metabolism. Fasting and CLA treatment of animals causes an increase in the amount of PE in endoplasmic reticulum (ER) membranes and mitochondria, as well as in the PE/phosphatidylcholine (PC) ratio. Moreover, the activity of the ethanolamine-specific phospholipid base exchange (PLBE) enzyme in liver ER membranes of fasted animals was enhanced by 75% in comparison to that of animals fed ad libitum. The effect of CLA treatment was additive to that of starvation; PE synthesis tested in vitro via the Ca2+-sensitive PLBE reaction increased 3-fold in comparison to rats fed ad libitum. This is confirmed by an increased Vmax for the reaction, but the affinity of the enzyme for ethanolamine was not significantly changed. These effects were accompanied by an enhanced expression of cytochrome P450 CYP4A1 isoform and elevated activity of the enzyme upon CLA administration. The stimulatory effect of CLA administration on the efficiency of the ethanolamine-specific PLBE reaction can be explained by elimination of lauric acid, a known inhibitor of de novo PE synthesis, during the course of omega-hydroxylation catalysed by CYP4A1, and by increased expression of the PLBE enzyme. The products of omega-hydroxylation of lauric acid, which are then converted by dehydrogenase to 1,12-dodecanedioic acid, did not significantly affect the in vitro synthesis of PE.

Differential expression of the peroxisome proliferator-activated receptor gamma (PPARgamma) and its coactivators steroid receptor coactivator-1 and PPAR-binding protein PBP in the brown fat, urinary bladder, colon, and breast of the mouse

Peroxisome proliferator-activated receptors (PPARs) regulate genes involved in lipid metabolism and adipocyte differentiation. Steroid receptor coactivator-1 (SRC-1) and PPAR-binding protein (PBP) interact with PPARgamma and act as coactivators to enhance ligand-dependent transcription. We report here that PPARgamma, SRC-1, and PBP are differentially expressed in the brown fat, transitional epithelium of the urinary bladder, colonic mucosa, and mammary epithelium of the adult mouse. PPARgamma and PBP are expressed in the transitional epithelium of urinary bladder and in brown adipose tissue, but not SRC-1. In the colonic mucosa, PPARgamma expression occurs throughout the villi, whereas the expression of both SRC-1 and PBP is confined mostly to the crypts. The expression of both SRC-1 and PBP is prominent in the breast epithelium of nonpregnant, pregnant, and lactating mice, whereas PPARgamma expression appeared prominent during lactation. During early embryonic development, PPARgamma, SRC-1, and PBP are differentially expressed, with only limited cell types displaying overlapping expression. PPARgamma and PBP expression overlapped in the brown fat and urogenital sinus at stage E15.5 of embryogenesis, whereas SRC-1 expression occurred mostly in neuroepithelium and cartilage between stages E9.5 and E13.5 of embryogenesis.

Mechanisms of peroxisome proliferation by perfluorooctanoic acid and endogenous fatty acids

1. The effects of endogenous fatty acids and perfluorooctanoic acid (PFOA) and its analogs on peroxisomal acyl CoA oxidase (ACO) and microsomal laurate hydroxylase (LH) activities were evaluated in primary cultures of rat hepatocytes and activation of peroxisome proliferator-activated receptor alpha (PPARalpha) in CV-1 cells. The rank order for the stimulation of ACO activity in hepatocytes for selected compounds was PFOA >> octanoic acid>octanedioic acid, perfluorooctanol (inactive). Increases in ACO activity by PFOA, like those of ciprofibrate, were associated with a marked increase in peroxisome number and cytosolic occupancy volume. Maximal effects of ciprofibrate and PFOA on the stimulation of ACO activity were not additive, suggesting that these two compounds share a common pathway of peroxisome proliferation. 2. Saturated monocarboxylic acids of C4 to C18 chain length were inactive, and, among dicarboxylic acids, only small elevations (40-45%) in ACO activity were observed with the long-chain C12 and C16 dioic acids. Of the C18 fatty acids tested, only oleic and linoleic acids, at 1 mM, produced a two- to three-fold elevation in ACO and LH activities. In comparison with endogenous fatty acids, PFOA was more potent and exhibited a different time course and greater magnitude of stimulation of ACO and LH activities in cultured hepatocytes. 3. Addition of mitochondrial beta-oxidation inhibitors (3-mercaptopropionic and 2-bromooctanoic acids) did not alter ACO activity in the presence of octanoic acid or octanedioic acid; nor did they modify the stimulation of ACO activity by PFOA. The carnitine palmitoyltransferase I inhibitor 2-bromopalmitic acid produced a 2.5-fold increase in ACO stimulatory activity and reduced both ciprofibrate- and PFOA-mediated stimulations of ACO activity. 4. Cycloheximide treatment reduced PFOA- and ciprofibrate-induced ACO activities; however, the response to oleic acid was not blocked and increased slightly. 5. In rat and human PPARalpha transactivation assays, the rank order of activation was ciprofibrate > PFOA > oleic acid > or = octanoic acid > octanedioic acid or perfluorooctanol (inactive). PFOA, ciprofibrate and oleic acid were activators of rPPARalpha at concentrations that correlated favorably with the changes in ACO activity in cell culture. Octanoic acid did not increase ACO activity and was a weak activator of PPARalpha. 6. Our findings suggest that fatty acids such as oleic acid (endogenous fatty acids) and PFOA (a stable fatty acid) act through more than one pathway to increase ACO activity in rat hepatocytes. We conclude that the potent effects of PFOA are primarily mediated by a mechanism that includes the activation of liver PPARalpha.

Hepatocyte apoptosis and hepatic expression of transforming growth factor-beta1 mRNA during involution of hyperplastic rat liver induced by hepatocyte growth factor

Hepatocyte apoptosis occurs during involution of hyperplastic liver induced by administration of xenobiotic compounds in rats. With this hyperplasia and involution, hepatic transforming growth factor (TGF)-beta1 is reported to be expressed to stimulate hepatocyte apoptosis. In regenerating liver after partial resection showing no hyperplasia, such expression of TGF-beta1 is also seen. However, no hepatocyte apoptosis develops despite the high levels of TGF-beta1. When rats received an intravenous injection of human hepatocyte growth factor at 12 h intervals for 14 days, the hepatic DNA content was increased 12 h after the last injection to 140% of control. This DNA content was significantly decreased at 108 and 180 h after discontinuation of treatment. At 60 h after the last injection, the number of apoptotic bodies positive for nick end-labelling of DNA in hepatocytes was significantly greater in treated rats than in control rats. Hepatocyte apoptosis was also identified electron micrographically. Hepatic TGF-beta1 mRNA levels in treated rats were significantly lower than in control rats at 12 h and then gradually increased towards control levels. We conclude that hyperplastic liver induced in normal rats by hepatocyte growth factor regresses with hepatocyte apoptosis and suppressed hepatic TGF-beta1 mRNA levels.

Differential effects of nongenotoxic and genotoxic carcinogens on the preneoplastic lesions in the rat liver

Glutathione S-transferase placental form (GST-P) positive foci development and its expression in liver exposed by nongenotoxic carcinogens phenobarbital (PB) and clofibrate (CF), and genotoxic carcinogen 2-amino-3-methylimidazo[4,5-f] quinoline (IQ) were investigated as a measure of carcinogenic potential of these chemicals. Male F344 rats were initially given a single intraperitioneal injection of diethylnitrosamine (200 mg/kg), and 2 weeks later, animals were fed diets containing 0.03% IQ or 0.5% CF or 0.05% PB or basal diet as a control for 6 weeks. All rats were subjected to two-thirds partial hepatectomy (PH) at week 3. Sequential sacrifice of rats was performed at 8 weeks or 52 weeks, and liver tissues were examined for immunohistochemical staining of GST-P positive foci. The numbers (No./cm2) and areas (mm2/cm2) of GST-P positive foci were increased by IQ or PB, but were decreased by CF compare to the control. Consistent with the development of GST-P positive foci, a time-related increase in the expression of GST-P mRNA was found in the rats treated with IQ, whereas CF decreased it. The incidence of hepatocellular carcinoma at 52 weeks was increased by all three chemicals. These results show that PB and IQ induced GST-P positive foci, but the peroxisome proliferator CF did not, which suggest that the prediction of carcinogenic potency based on the development of prenoplastic foci may cause false negative in a particular category compounds like peroxisome proliferators.

Peroxisome proliferators induce apoptosis in hepatoma cells

In the AH-130 hepatoma, a poorly differentiated tumor, maintained by weekly transplantations in rats, a low percentage of cells spontaneously underwent apoptosis, mainly during the transition from logarithmic- to stationary-growth phase. It was possible to induce massive apoptosis of cells by treating them with clofibrate, a peroxisome proliferator and hypolipidemic drug. Similar results were obtained with HepG2 cells. With 1 mM clofibrate, apoptosis began to manifest itself after 1 h of treatment in vitro, and was assessed by morphological analysis, by DNA fragmentation carried out with agarose gel electrophoresis, and with flow cytometric determination of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling. The mechanisms whereby clofibrate induces apoptosis are still unclear. Since the peroxisome proliferator-activated receptor was expressed at a very low level and was not stimulated by clofibrate in the AH-130 hepatoma cells, its involvement seems unlikely. Moreover, lipid peroxidation was not increased after clofibrate treatment. Phospholipids and cholesterol were significantly decreased. The decreased cholesterol content might suggest an inhibition of the mevalonate pathway and, therefore, of isoprenylation of proteins involved in cell proliferation.

Steatohepatitis, spontaneous peroxisome proliferation and liver tumors in mice lacking peroxisomal fatty acyl-CoA oxidase. Implications for peroxisome proliferator-activated receptor alpha natural ligand metabolism

Peroxisomal beta-oxidation system consists of four consecutive reactions to preferentially metabolize very long chain fatty acids. The first step of this system, catalyzed by acyl-CoA oxidase (AOX), converts fatty acyl-CoA to 2-trans-enoyl-CoA. Herein, we show that mice deficient in AOX exhibit steatohepatitis, increased hepatic H2O2 levels, and hepatocellular regeneration, leading to a complete reversal of fatty change by 6 to 8 months of age. The liver of AOX-/- mice with regenerated hepatocytes displays profound generalized spontaneous peroxisome proliferation and increased mRNA levels of genes that are regulated by peroxisome proliferator-activated receptor alpha (PPARalpha). Hepatic adenomas and carcinomas develop in AOX-/- mice by 15 months of age due to sustained activation of PPARalpha. These observations implicate acyl-CoA and other putative substrates for AOX, as biological ligands for PPARalpha; thus, a normal AOX gene is indispensable for the physiological regulation of PPARalpha.

Effect of rodent hepatocarcinogenic peroxisome proliferators on fatty acyl-CoA oxidase, DNA synthesis, and apoptosis in cultured human and rat hepatocytes

The effects of the rodent hepatocarcinogens clofibric acid and diprofibrate on the activity of the peroxisomal fatty acyl-CoA oxidase, DNA synthesis, and apoptosis were compared in cultured rat and human hepatocytes. Rat hepatocytes expressed a 10-fold greater level of the peroxisomal fatty acyl-CoA oxidase compared to human hepatocytes. At the highest concentration (1.0 mM), both drugs induced a two- to threefold increase in this enzyme activity in both rat and human hepatocytes. Ciprofibrate (0.1 and 0.2 mM) caused a twofold increase in DNA synthesis in rat hepatocytes, whereas clofibric acid had no effect on DNA synthesis in these cells. In contrast, increasing concentrations of both clofibric acid and ciprofibrate produced inhibition of DNA synthesis in human hepatocytes. By using the terminal transferase dUTP-biotin nick end labeling technique, it was observed that 0.1 and 0.2 mM clofibric acid and ciprofibrate suppressed transforming growth factor-beta (TGF beta)-induced apoptosis by 50% in rat hepatocytes, but they had no effect on TGF beta-induced apoptosis in human hepatocytes. Although clofibric acid and ciprofibrate diminished TGF beta-induced apoptosis, they had no effect on the basal apoptotic levels in the rat hepatocyte cultures. However, both drugs significantly increased the percent of apoptotic cells in the human hepatocyte cultures. It is concluded that primary rat and human hepatocyte cultures respond differently to peroxisome proliferators. The differences in effects on DNA synthesis and apoptosis support the hypothesis that human liver cells are refractory to peroxisome proliferator-induced hepatocarcinogenesis.

Nephrotoxic effects of di-(2-ethylhexyl)-phthalate (DEHP) hydrolysis products on cultured kidney epithelial cells

1. Di-(2-ethylhexyl)-phthalate (DEHP) possesses a great industrial value as a plasticizing agent and has become an ubiquitous environmental contaminant. In most species it is rapidly metabolized to mono-(2-ethylhexyl)-phthalate (MEHP) and 2-ethylhexanoic acid (2-EHA). Evaluation of toxicity of DEHP and its primary metabolites has been focussed on reproductive toxicity and hepatocarcinogenic properties. The aim of this study was to determine the nephrotoxic potential of both DEHP metabolites by use of cultured kidney epithelial cells (Opossum kidney cells; OK cells). 2. For this purpose, OK cells were exposed for 3 days to MEHP and 2-EHA at concentrations ranging from 0.1 -500 micromol/L and the toxicity as well as the effects on migratory activity and intracellular cytoskeleton were studied by cell biological, morphological and morphometric methods. 3. When compared with corresponding controls, treatment of OK cells with MEHP and 2-EHA, respectively, showed marked differences in cell viability between both DEHP metabolites. MEHP caused a dose-dependent decrease in cell viability (ED50 = 25 micromol/L) accompanied by a moderate swelling of the cells at concentrations up to 25 micromol/L. MEHP concentrations higher than 25 micromol/L caused a dose-dependent shrinkage of the cells and the occurrence of a high amount of cell debris as a result of cell lysis. 2-EHA did not cause a reduced viability or an altered cell volume. The migratory activity of OK cells was not significantly influenced by both metabolites. Moreover, MEHP toxicity resulted in a largely reduced and altered organization of F-actin (stress fibers), but not of myosin, microtubules and vimentin. 4. The study indicates that cultured epithelial cells can be used as a prescreening system to assess the nephrotoxicity of hazardous substances such as DEHP. As demonstrated in this study, only MEHP, but not 2-EHA, has a marked nephrotoxic effect in vitro.

Effect of the peroxisome proliferator ciprofibrate on lipid peroxidation and 8-hydroxydeoxyguanosine formation in transgenic mice with elevated hepatic catalase activity

Peroxisome proliferators are a group of non-genotoxic hepatic carcinogens which have been proposed to act by increasing oxidative damage in the liver. To test this hypothesis, we have produced a transgenic mouse line that has elevated catalase activity specifically in the liver. In this study, we have examined if catalase overexpression influences the induction of lipid peroxidation or oxidative DNA damage, two mechanisms which have been hypothesized to be important in the carcinogenesis by peroxisome proliferators. Transgenic mice or non-transgenic litter mates were fed either 0.01% ciprofibrate or a control diet for 21 days. The activities of fatty acyl CoA oxidase and lauric acid hydroxylase were not significantly affected by catalase overexpression, although the ratio of fatty acyl CoA oxidase to catalase was significantly decreased in transgenic animals. Hepatic lipid peroxidation was estimated by quantifying the concentrations of malondialdehyde and conjugated dienes. Ciprofibrate treatment did not affect either endpoint, but catalase overexpression increased the concentrations of malondialdehyde (in untreated mice only) and conjugated dienes (in both untreated and ciprofibrate-fed mice). Oxidative DNA damage was estimated by quantifying 8-hydroxydeoxyguanosine (8-OHdG) by high-performance liquid chromatography/electrochemical detection. Ciprofibrate treatment significantly increased hepatic 8-OHdG concentrations, in agreement with several previous studies, but catalase overexpression did not significantly affect them, although 8-OHdG concentrations were decreased 50% in untreated mice. These results imply that the metabolism of hydrogen peroxide by catalase is not an important factor in the development of hepatic lipid peroxidation. The decrease in hepatic 8-OHdG in untreated transgenic mice and the increase seen after ciprofibrate administration imply that hydrogen peroxide is important in the formation of 8-OHdG. While the lack of decreased 8-OHdG levels in ciprofibrate-treated transgenic mice does not support this conclusion, it is possible that catalase levels were not sufficiently high to affect this endpoint. Transgenic mice with higher hepatic catalase activities may be required to resolve this issue.

Induction of hepatic poly(ADP-ribose) polymerase by peroxisome proliferators, non-genotoxic hepatocarcinogens

Two peroxisome proliferators, [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid (Wy-14,643) or di(2-ethylhexyl) phthalate (DEHP), were given orally to male F-344 rats for up to 78 or 97 weeks. At 1 week, the activity of poly(ADP-ribose) polymerase (pADPRP) was increased 2- and 1.8-fold in the liver of rats treated with Wy-14,643 and DEHP, respectively. The induction of the activity was maintained at 2.5- or 2-fold for up to 52 weeks. The immunoblot and Northern blot analyses revealed that the induction of pADPRP activity would be responsible for the increase in the amount of mRNA. In addition, in the liver tumor induced by Wy-14,643 and DEHP, the pADPRP mRNA level increased 3.6- or 3.7-fold. The magnitude of the increase in the mRNA level was higher than that in the non-tumor portion. These findings suggest that the induction of pADPRP may play an important role in the hepatocarcinogenesis induced by peroxisome proliferators.

Repeated dosing with the peroxisome proliferator clofibrate decreases the toxicity of model hepatotoxic agents in male mice

Pretreatment of mice with clofibrate (CFB) has been shown to protect against acetaminophen (APAP) hepatotoxicity. To determine if pretreatment with CFB prevents the toxicity of other model hepatotoxicants, male C57BL6J or CD-1 mice received 500 mg CFB/kg, i.p., daily for 10 days, and then were challenged with either 250 mg bromobenzene (BrB)/kg, 0.025 ml carbon tetrachloride (CCl4)/kg or 0.5 ml chloroform (CHCl3)/kg. Liver and kidney injury was assessed by plasma sorbitol dehydrogenase activity (SDH) and blood urea nitrogen (BUN), respectively and histopathology. Challenge with BrB significantly elevated plasma SDH activity in C57Bl6J mice. This was prevented in CFB pretreated mice receiving the same dose of BrB. Changes in BUN were not detected in either group of BrB treated mice. Similarly, pretreatment of male CD-1 mice with CFB significantly reduced CCl4-induced elevation in plasma SDH activity, with no BUN elevation detected in either group. CFB pretreatment also diminished elevation in plasma SDH activity produced by CHCl3 in CD-1 mice, while BUN was significantly elevated in both groups, indicating that CFB did not protect against CHCl3-induced nephrotoxicity. Histopathological examination of liver and kidney sections confirmed these results. This study shows that mice pretreated with CFB were protected from toxicity at 24 h after challenge with other model hepatotoxic agents besides APAP.

Inhibition of PPAR alpha/RXR alpha-mediated direct hyperplasia pathways during griseofulvin-induced hepatocarcinogenesis

Chronic griseofulvin (GF) feeding induces preneoplastic foci followed by hepatocellular carcinoma in the mouse liver. Our previous study suggested that GF-induced hepatocellular proliferation had a different mechanism from that of peroxisome proliferator (PP)-induced direct hyperplasia. The GF-induced hepatocellular proliferation was mediated through activation of immediate early genes such as Fos, Jun, Myc, and NFKB. In contrast, PP-induced direct hyperplasia does not involve activation of any of these immediate early genes. It has been shown that nuclear hormone receptors including peroxisome proliferator activated receptors (PPARs) and retinoid x receptors (RXRs) play important roles in mediating the pleiotropic effects of PPs. To examine the possible roles of PPARs and RXRs during non-PP-induced hepatocellular proliferation and the interaction between PP and non-PP-induced proliferation, we have studied the expression of the PPAR and RXR genes in the GF model using northern blot hybridizations and gel retardation assays. The data showed that the expression of PPARalpha and RXRalpha genes was down-regulated in the livers containing preneoplastic nodules and in the liver tumors induced by GF. The mRNA down-regulation was accompanied by a decrease in the amount of nuclear protein-bound to peroxisome proliferator and retinoic acid responsive elements. Down-regulation was also associated with the suppressed expression of the PPARalpha/RXRalpha target genes (i.e., acyl-Co oxidase and cytochrome P450 4A1) and the catalase gene. The RXR-gamma gene was also down-regulated, but the RARalpha, beta, and gamma and PPARbeta and gamma genes were up-regulated. These results indicated that the hepatocarcinogenesis induced by GF is accompanied by suppression of the PPARalpha/RXRalpha-mediated direct hyperplasia pathway. The differential expression of these nuclear hormone receptors reveals a new aspect for understanding the individual roles and intercommunication of PPAR, RXR, and RAR isoforms in the liver.

Effects of the peroxisome proliferator ciprofibrate and prostaglandin F2 alpha combination treatment on second messengers in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolizes lipids, including eicosanoids. Peroxisome proliferators transiently induce increased cell proliferation in vivo. However, peroxisome proliferators are weakly mitogenic and are not co-mitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Earlier study found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. In order to study possible mechanisms of the comitogenicity of peroxisome proliferator ciprofibrate and eicosanoids, we hypothesized that the co-mitogenicity may result from synergistic or additive increases of second messengers in mitogenic signal pathways. We therefore examined the effect of the peroxisome proliferator ciprofibrate, prostaglandin F2 alpha (PGF2 alpha) and the combination of ciprofibrate and PGF2 alpha with or without growth factors on the protein kinase C (PKC) activity, and inositol-1, 4, 5-triphosphate (IP3) and intracellular calcium ([Ca2+]i) concentrations in cultured rat hepatocytes. The combination of ciprofibrate and PGF2 alpha significantly increased particulate PKC activity. The combination of ciprofibrate and PGF2 alpha also significantly increased EGF, transforming growth factor-alpha (TGF-alpha) and hepatic growth factor (HGF)-induced particulate PKC activity. The combination of ciprofibrate and PGF2 alpha greatly increased [Ca2+]i. However, the increases of PKC activity and [Ca2+]i by ciprofibrate and PGF2 alpha alone were much smaller. Neither ciprofibrate or PGF2 alpha alone nor the combination of ciprofibrate and PGF2 alpha significantly increased the formation of IP3. The combination of ciprofibrate and PGF2 alpha, however, blocked the inhibitory effect of TGF-beta on particulate PKC activity and formation of IP3 induced by EGF. These results show that co-mitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from the increase in particulate PKC activity and intracellular calcium concentration but not from the formation of IP3.

Sex differences in dehydroepiandrosterone-induced hepatocarcinogenesis in the rat

Dehydroepiandrosterone (DHEA), a steroid secreted by the adrenal gland, is a peroxisome proliferator and a hepatocarcinogen. Previously, we have shown that 15-week-old male rats given DHEA in AIN-76 diet without vitamin E developed liver tumors. In the present study, we have examined the carcinogenic effect of DHEA in 5-6-week-old male rats and in intact and ovariectomized female rats. Rats were fed Purina chow containing DHEA at a concentration of 0.45% for 100 weeks and livers were evaluated for tumor incidence and multiplicity. In male rats the incidence of total tumors and hepatocellular carcinomas (HCC) was 94 and 81%, respectively, with 1.9 +/- 0.3 (mean +/- SD) tumors per liver. In intact and ovariectomized females the total tumor incidence was 46 and 60%, respectively. However, the incidence of HCC and the mean number of tumors were similar in both intact and ovariectomized groups. Phenotypically the neoplastic nodules and HCC in female rats were negative for gamma-glutamyltranspeptidase. The results of these studies further confirm that DHEA is a hepatocarcinogen in male and female rats. The possible reasons for the decreased incidence of liver tumors in females in relation to the peroxisome proliferative effect of DHEA is discussed.

Rapid one-step isolation of mouse liver catalase by immobilized metal ion affinity chromatography

A novel and rapid procedure for the isolation of catalase from mouse liver, after prior treatment with the peroxisome proliferator perfluorooctanoic acid was developed using immobilized metal ion affinity chromatography involving chelation with zinc ions. The purification developed is simple, rapid (requiring 3 hours from cytosol or peroxisomal matrix to homogeneous proteins), reproducible, and yields virtually complete overall recovery of catalase activity. This procedure makes catalase from a variety of tissues and physiological and environmental conditions more readily available for study.

Induction of peroxisomal oxidases in mussels: comparison of effects of lubricant oil and benzo(a)pyrene with two typical peroxisome proliferators on peroxisome structure and function in Mytilus galloprovincialis

Marine mussels are used as bioindicators of water pollution in marine and estuarine environments in the so-called "Mussel Watch" programs because of their capacity to accumulate numerous organic xenobiotics including aromatic hydrocarbons. In this study, we have analyzed the effects of two xenobiotics [benzo(a)pyrene and the water accommodated fraction of a lubricant oil] and two typical (rodent) peroxisome proliferators (clofibrate and dioctyl phthalate) on structure and function of peroxisomes in digestive glands of mussels Mytilus galloprovincialis, either following water exposure (for 1, 7, and 21 days) or after direct injection through the adductor muscle (for 1 and 7 days). The activities of catalase (CAT), acyl-CoA oxidase (AOX), and D-amino acid oxidase were determined in whole homogenates of digestive glands. In addition, stereological methods were applied on sections stained histochemically for demonstration of catalase activity in order to quantify the morphological changes of peroxisomes. The peroxisomal acyl-CoA oxidase and D-amino acid oxidase were increased in mussels injected for 7 days with benzo(a)pyrene, phthalate, and clofibrate and a similar trend was noted for benzo(a)pyrene and lubricant oil in water exposure experiments (21 days). The catalase activity was reduced or unchanged depending on the mode of exposure of animals. By stereology, significant increases of numerical and volume densities of peroxisomes were found in animals injected for 7 days with lubricant oil or clofibrate. These observations indicate that peroxisomal oxidases in mussels are induced at moderate rates in response to different xenobiotics and that their determination could provide a (sensitive) marker for detection of effects of some toxic pollutants, particularly the lubricant oils which in addition induce significant structural alterations of mussel peroxisomes.

Receptor and nonreceptor-mediated organ-specific toxicity of di(2-ethylhexyl)phthalate (DEHP) in peroxisome proliferator-activated receptor alpha-null mice

The peroxisome proliferator-activated receptor alpha (PPAR alpha) is the mediator of the biological effects of peroxisome proliferators through control of gene transcription. To determine if the toxic effects of di(2-ethylhexyl)phthalate (DEHP) are mediated by PPAR alpha, we examined its effect in PPAR alpha-null mice. Male Sv/129 mice, PPAR alpha-null (-/-) or wild-type (+/+) were fed ad libitum either a control diet or one containing 12,000 ppm DEHP for up to 24 wk. Significant body weight loss and high mortality was observed in (+/+) mice fed DEHP. By 16 wk, all DEHP-fed (+/+) mice had died of cystic renal tubular disease. In contrast, the (-/-) mice fed DEHP had no changes in body weight until later in the study nor increased mortality. Histologically, (+/+) mice fed DEHP had typical toxic lesions in liver, kidney, and testis while (-/-) mice fed DEHP had no toxic liver lesions but did show evidence of toxicity in kidney and testis after 4-8 wk of feeding, which progressed into moderate lesions by 24 wk. Analysis of hepatic and renal mRNAs showed a typical pleiotropic response in gene expression in the DEHP-fed (+/+) mice that was absent in the DEHP-fed (-/-) mice. These results provide evidence that PPAR alpha mediates the subacute-chronic toxicity of DEHP in liver, kidney, and testis. However, because (-/-) mice did develop toxic lesions in kidney and testis, DEHP can also act through PPAR alpha-independent pathways in mediating renal and testicular toxicity.

Effect of the peroxisome proliferator ciprofibrate on hepatic cyclooxygenase and phospholipase A2 in rats

Peroxisome proliferators, which include several hypolipidemic drugs, plasticizers and other chemicals, induce hepatic tumors in rodents. These chemicals alter the expression of enzymes involved in lipid metabolism, such as the cytochrome P450 4A family and peroxisomal beta-oxidation enzymes. Previous studies have shown that the peroxisome proliferator ciprofibrate reduces eicosanoid concentrations in rat livers and primary hepatocyte cultures, yet the mechanism is still unclear. In this study we examined cyclooxygenases 1 and 2 (COX-1 and COX-2) and cytosolic phospholipase A2 (cPLA2) to determine whether the rate-limiting enzymes in the eicosanoid synthetic pathway are altered by ciprofibrate. Rats were fed 0.01% ciprofibrate for 3, 6, or 10 days. Western analysis revealed that COX-2 protein was induced by ciprofibrate (up to 13-fold at day 10), but that calcium-dependent (Ca-D) cPLA2 protein was not different from controls. The enzyme activity of calcium-independent (Ca-I) cPLA2 in ciprofibrate-treated rats was increased 2-fold, whereas Ca-D cPLA2 and total COX activities were not affected. Using enzyme kinetics, we found that COX-1 (Ki = 143 microM) and Ca-I cPLA2 (Ki = 121 microM) were competitively inhibited by ciprofibrate, but the inhibition was not physiologically significant. COX-2 and Ca-D cPLA2 were not inhibited by ciprofibrate. These results show that ciprofibrate increases Ca-I cPLA2 enzyme activity and COX-2 protein expression.

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response. Copyright 1998 Academic Press.

Monitoring of PPAR alpha protein expression in human tissue by the use of PPAR alpha-specific MAbs

We report the production and characterization of two PPAR alpha subtype-specific monoclonal antibodies raised against the N-terminal domain of PPAR alpha. P alpha b 11.80A is a Western-reactive antibody, whereas P alpha b 32.51 is useful for immunohistochemistry. Both antibodies exhibited high affinity against the immunogen based on BIAcore analysis, recognized full-length PPAR alpha protein in PPAR alpha-transfected CV-1 cells, and displayed no cross-reactivity against the N-terminal domains of PPAR gamma or PPAR delta proteins as demonstrated by various immunoassays. The application of these antibodies to a panel of normal human tissues revealed that PPAR alpha protein expression is highest in skeletal muscle, liver, and kidney, consistent with previously reported mRNA expression data. These antibodies provide us with valuable tools to further explore the function of PPAR alpha.

Isolation of human CYP4F2 genomic DNA and its 5' end regulatory region structure

Human cytochrome P450 4F2 shows high regioselectivity in omega-hydroxylation of stearic acid and leukotriene B4. As a first step of its regulation study, human cytochrome P450 4F2 genomic DNA was isolated from liver of a person who was administered clofibrate for 10 years. From Southern hybridization, restriction enzyme digestion and sequencing experiments, isolated genomic DNA fragment was found to contain around 32 Kb DNA and more than 20 Kb of 5' end regulatory region. Sequences of the structural gene region revealed exon 1 and exon 2. Further regulation studies would elucidate the feedback mechanisms of the oxidative degradation of fatty acids, inflammatory response and the clearance of leukotriene B4 in the liver. Furthermore, regulation study of this gene could explain the species difference in response to peroxisome proliferator and help in the safety evaluation of peroxisome proliferating chemicals to human being.

Endocrine treatment of hepatocellular carcinoma. Any evidence of benefit?

In the past 20 years, a number of studies have investigated the relationship between sex hormones and liver cancer. Experimental studies indicate that a dynamic process, with sequential modifications in the pattern of sex hormones in the serum and of sex hormone receptors in the liver, occurs progressively during hepatocarcinogenesis. Overall, it seems that both androgens and oestrogens may enhance liver carcinogenesis, while androgens may also support the growth of established liver tumours. Unfortunately, clinical studies of endocrine treatment of hepatocellular carcinoma (HCC) have not adequately tested the suggestions from biological studies. So far, no clinical trial has been performed to test the efficacy of endocrine manipulation for the chemoprevention of HCC in cirrhotic patients nor in preventing relapse after radical resection of primary HCC. Anti-oestrogens have been the most studied agents for the endocrine treatment of established HCC, although the rationale that supports their use is weaker than for anti-androgens. Studies with anti-androgens have produced prevalently negative results, due to either a lack of activity or excessive toxicity. The use of chemical castration, which theoretically could enhance the activity of antihormonal compounds, yielded no benefit at all. In summary, there is, as yet, no definitive evidence that endocrine treatment favourably affects the outcome of patients with HCC.

A glucose-regulated protein, GRP58, is down-regulated in C57B6 mouse liver after diethylhexyl phthalate exposure

Diethylhexyl phthalate (DEHP) is a widely used plasticizer that induces peroxisome proliferation in rodents. Prolonged exposure to DEHP results in a variety of toxic effects, the most significant of which appears to be an increased incidence of liver cancer and male reproductive toxicity in rodents. Accompanying these toxic effects is the induction of a number of genes within the liver, particularly those genes involved in peroxisomal fatty acid beta-oxidation and members of the cytochrome P450 family, CYP4A. In order to explore which additional genes may be altered by DEHP exposure, mRNA differential display was performed using total liver RNA from male C57B6 mice that were treated with either O or 2% DEHP in their diet for 7 days. In doing so, a number of partial cDNAs representing messages that are potentially differentially expressed have been isolated. One of these cDNAs was found to be similar to the previously cloned gene, GRP58. Analysis by RNase protection assay and North hybridization have shown that the transcript for GRP58 is down-regulated in the liver after DEHP exposure. Analysis of dose-response exposures to DEHP by reverse transcription (RT)-PCR confirm these results and also shows that GRP58 is not altered in kidney or testis. Immunoblot analysis using GRP58-specific antibodies also shows a decrease in GRP58 protein levels in DEHP-treated mice. Moreover, exposure of mice to another peroxisome proliferator, clofibrate, results in a slight down-regulation of GRP58 at the highest dose, 0.5%. Thus, it appears as if DEHP and clofibrate can use different pathways to affect gene expression.

Comparison of the effects of di-(2-ethylhexyl)adipate on hepatic peroxisome proliferation and cell replication in the rat and mouse

The effects of di-(2-ethylhexyl)adipate (DEHA) have been compared in female F344 rats and female B6C3F1 mice fed diets containing 0-4.0% DEHA and 0-2.5% DEHA, respectively, for periods of 1, 4 and 13 weeks. In both the rat and mouse treatment with DEHA at all time points produced a dose-dependent increase in relative liver weight and hepatic peroxisome proliferation as demonstrated by the induction of peroxisomal (cyanide-insensitive palmitoyl-CoA oxidation) and microsomal (lauric acid 12-hydroxylase) fatty acid oxidising enzyme activities. The magnitude of induction of peroxisome proliferation was similar in both species. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during study weeks 0-1, 3-4 and 12-13. After 1 week DEHA treatment hepatocyte labelling index values were increased in rats given 2.5 and 4.0% DEHA and mice given 0.6-2.5% DEHA. While DEHA treatment for 4 and 13 weeks did not increase labelling index values in the rat, a sustained stimulation of replicative DNA synthesis was observed in mice given 1.2 and 2.5% DEHA. The results of this study demonstrate a species difference in the hepatic effects of DEHA, in that at some dose levels DEHA can produce a sustained stimulation of replicative DNA synthesis in mouse but not in rat liver. Sustained cell replication provides a better correlation with the observed formation of liver tumours in chronic studies with DEHA in female mice, but not in female rats, than the magnitude of stimulation of hepatic peroxisome proliferation.

Lack of correlation between hepatic prostaglandin concentrations and DNA synthesis after the administration of phenobarbital and the peroxisome proliferator ciprofibrate in rats

Peroxisome proliferators are a class of chemicals that induce and promote hepatic tumors in rodents. These compounds are not genotoxic, and the mechanism by which they induce and promote tumors is poorly understood. Phenobarbital (PB) also is a hepatic tumor promoter that produces a different natural history than peroxisome proliferators during the promotion of hepatocarcinogenesis. In addition, opposite effects on hepatic eicosanoid concentrations have been demonstrated previously. In this experiment, we examined whether higher hepatic eicosanoid concentrations correlated with the induction of DNA synthesis after the administration of PB or the peroxisome proliferator ciprofibrate (CIP). PB (0.05% in diet) or CIP (0.01% in diet) was fed to rats from 1-10 days. For the rats treated with CIP, the peroxisomal enzyme fatty acyl-CoA oxidase increased gradually from day 1 to day 10. PB treated rats had a higher cytochrome P450 2B1/2 activity over the entire course of feeding. Hepatic prostaglandins E2 and F2alpha concentrations were significantly reduced in the rats treated with CIP, while no significant differences were seen between the control and PB-treated rats. DNA synthesis was increased in both PB-treated and CIP-treated rats. These results show that higher eicosanoid concentrations do not correlate with the induction of hepatic DNA synthesis by CIP or PB.

Regulation of peroxisomal fatty acyl-CoA oxidase in the yeast. Saccharomyces cerevisiae

Peroxisomes are specialized organelles found in most eukaryote cells, where their major functions are in cellular respiration and fatty acid oxidation. Proliferation of this organelle, and induction of peroxisomal enzymes, is a phenomenon that occurs in diverse species, and is stimulated by a number of physiological and pharmacological stimuli. A large number of chemically diverse compounds, including hypolipidemic drugs and industrial plasticizers, have been shown to cause peroxisome proliferation and the induction of peroxisomal enzymes in rodents. Chronic exposure to these compounds produces hepatocellular carcinomas, however, the mechanism by which this tumorigenic event occurs is unknown. In the yeast Saccharomyces cerevisiae peroxisomes are induced when a fatty acid such as oleate is supplied as a carbon source in the growth medium. In addition, many peroxisomal enzymes are induced by growth on oleate; these include enzymes of the peroxisomal beta-oxidation cycle. This regulation occurs at the transcription level, and is controlled by specific trans-acting factors. The research in our laboratory has focused on the mechanisms involved in this regulation, and on the identification and characterization of the proteins involved. Our recent results, and current research directions are summarized.

Dietary fat, genes, and human health

These studies show that a macronutrient like dietary fat plays an important role in gene expression. In the cases presented here, dietary fat regulates gene expression leading to changes in carbohydrate and lipid metabolism. The interesting outcome of these studies is the finding that the molecular targets for dietary fat action did not converge with the principal targets for hormonal regulation of gene transcription, like hormone receptors. Instead, PUFA-RF targets elements that play key ancillary roles in gene transcription. This is important because it shows how PUFA can interfere with hormone regulation of a specific gene without having generalized effect on overall hormonal control, i.e. PUFA effects are promoter-specific. How PUFA-RF interferes with gene transcription will require the isolation and characterization of PUFA-RF along with the tissue-specific factors targeted by PUFA-RF. A different story emerges when fatty acids activate PPAR. Based on the studies presented here and elsewhere, long chain-highly unsaturated fatty acids (like 20:5,n-3 and 22:6, n-3) or high levels of fat activate PPAR. PPAR directly activates genes like AOX, but also inhibits transcription of genes like S14, FAS, apolipoprotein CIII, transferrin. For S14, the mechanism of inhibition involves sequestration of RXR, a critical factor for T3 receptor binding to DNA. Thus, PPAR can have generalized effects on T3 action or on other nuclear receptors, like vit. D (VDR) and retinoic acid (RAR) receptors, that require RXR for action. For apolipoprotein CIII and transferrin, PPAR/RXR heterodimers compete for HNF-4 binding sites (DR + 1). In addition to HNF-4, COUP-TF, ARP-1 and RXR all bind the DR + 1 type motif. These factors are important for tissue-specific regulation of gene transcription. PPAR can potentially interfere with the transcription of multiple genes through disruption of nuclear receptor signaling leading to changes in phenotype. Clearly, more studies are required to assess the role PPAR plays in the fatty acid regulation of gene transcription and its contribution to chronic disease. Finally, it is clear that dietary fat has the potential to affect gene expression through multiple pathways. Depending on the gene examined, PUFA might augment or abrogate gene transcription which leads to specific phenotypic changes altering metabolism, differentiation or cell growth. These effects can be beneficial to the organism, such as the n-3 PUFA-mediated suppression of serum triglycerides or detrimental, like the saturated and n-6 PUFA-mediated promotion of insulin resistance. How such effects contribute to the onset or progression of specific neoplasia is unclear. However, studies in metabolism might provide important clues for this connection.

Suppression of cytochrome P450 Cyp2f2 mRNA levels in mice by the peroxisome proliferator diethylhexylphthalate

Exposure to peroxisome proliferators, which are extensively used, causes a number of pleiotrophic effects. Prolonged exposure to the peroxisome proliferator, DEHP, causes hepatic hyperplasia and liver tumors in rats and mice. This exposure can also induce a number of enzymes. To identify additional genes that are regulated by DEHP, mRNA differential display was used. One of the genes affected is cytochrome 450 Cyp2f2, a naphthalene hydroxylase. Using northern analysis, RNase protection assay, and RT-PCR, we show that the Cyp2f2 mRNA levels are decreased in mouse liver following DEHP treatment. A smaller Cyp2f2 mRNA transcript was also detected in kidney and these transcript levels were also suppressed but to a lesser extent than that in the liver. The response to DEHP in mouse liver is both dose and time dependent.

Amino acid residues in both the DNA-binding and ligand-binding domains influence transcriptional activity of the human peroxisome proliferator-activated receptor alpha

We have investigated the basis of the lack of activity of a natural variant human peroxisome proliferator-activated receptor alpha, hPPARalpha6/29. A subcloning approach was used to change the four variant amino acids in the hPPARalpha6/29 sequence, individually and in combination, to those found in an active human PPARalpha. Individual amino acid "back mutations" were unable to confer on hPPARalpha6/29 the ability to be activated by peroxisome proliferators in a transient transfection assay. Although hPPARalpha6/29 was able to bind specifically to DNA in the presence of the retinoid X receptor alpha (RXRalpha), the complete restoration of receptor transcriptional activity required two separate back mutations of the hPPARalpha6/29 sequence, namely amino acid 123 in the DNA binding domain, and amino acid 444 close to the C-terminus. This suggests that sequences in the PPARalpha DNA binding domain influence other receptor functions besides DNA binding.

Peroxisome proliferator activated receptors in Atlantic salmon (Salmo salar): effects on PPAR transcription and acyl-CoA oxidase activity in hepatocytes by peroxisome proliferators and fatty acids

A cDNA fragment which encodes salmon peroxisome proliferator activated receptor y (sPPARgamma) was amplified by PCR from the liver of Atlantic salmon (Salmo salar L.). The fragment was 627 bp long. The sequence of the amplified PCR product was similar to the PPARgamma of mouse and hamster. 59% of the bases were identical. Northern blot analysis of salmon liver mRNA showed that the amplified sPPARgamma fragment hybridised to three specific transcripts of lengths 1.6, 2.4 and 3.3 kb. Clofibric acid and bezafibrate, administered to salmon hepatocytes in culture, resulted in a 1.7-fold increase of the 1.6 kb sPPARgamma transcript. The activity of acyl-CoA oxidase also increased approx. 1.7-fold after administration of fibrates. These results indicate that PPAR is an important factor in mediating enzymatic response to fibrates in fish.

Isolation and characterization of PBP, a protein that interacts with peroxisome proliferator-activated receptor

In an attempt to identify cofactors that could possibly influence the transcriptional activity of peroxisome proliferator-activated receptors (PPARs), we used a yeast two-hybrid system with Gal4-PPARgamma as bait to screen a mouse liver cDNA library and have identified steroid receptor coactivator-1 (SRC-1) as a PPAR transcriptional coactivator. We now report the isolation of a cDNA encoding a 165-kDa PPARgamma-binding protein, designated PBP which also serves as a coactivator. PBP also binds to PPARalpha, RARalpha, RXR, and TRbeta1, and this binding is increased in the presence of specific ligands. Deletion of the last 12 amino acids from the carboxyl terminus of PPARgamma results in the abolition of interaction between PBP and PPARgamma. PBP modestly increased the transcriptional activity of PPARgamma, and a truncated form of PBP (amino acids 487-735) acted as a dominant-negative repressor, suggesting that PBP is a genuine coactivator for PPAR. In addition, PBP contains two LXXLL signature motifs considered necessary and sufficient for the binding of several coactivators to nuclear receptors. In situ hybridization and Northern analysis showed that PBP is expressed in many tissues of adult mice, including the germinal epithelium of testis, where it appeared most abundant, and during ontogeny, suggesting a possible role for this cofactor in cellular proliferation and differentiation.

Dietary glycine prevents increases in hepatocyte proliferation caused by the peroxisome proliferator WY-14,643

Peroxisome proliferators are a group of nongenotoxic carcinogens which include a number of hypolipidemic drugs, solvents, and industrial plasticizers. Although the mechanism by which they cause cancer remains unknown, one likely possibility is that they act as tumor promoters by increasing cell proliferation. Hepatic Kupffer cells represent a rich source of mitogenic cytokines (e.g., tumor necrosis factor alpha, TNF alpha) and are stimulated by peroxisome proliferators. Since glycine prevents activation of Kupffer cells, these experiments were designed to test the hypothesis that a diet containing glycine could block the mitogenic effect of the peroxisome proliferator [[4-chloro-6-(2,3-xylidino)pyrimidinyl]thio]acetic acid (WY-14,643). The effects of a glycine-enriched diet on WY-14,643-induced increases in cell proliferation after a single dose or after feeding WY-14,643 in the diet for 3 weeks were assessed. As expected, 24 h after a single dose of WY-14,643, rates of cell proliferation increased from basal values of 0.7 +/- 0.3% to 5.1 +/- 0.5%. Glycine largely prevented the increase caused by WY-14,643 with proliferation only reaching 1.9 +/- 0.4% (p < 0.05). Acyl CoA oxidase increased from 1.4 +/- 0.1 to 3.5 +/- 0.6 nmol of H2O2 min-1 (mg of protein)-1 (p < 0.05) indicating that peroxisome-specific enzyme activity was induced about 2-fold in livers of WY-14,643-treated rats after 24 h. Unlike cell proliferation, however, acyl CoA oxidase was not affected by dietary glycine, consistent with the hypothesis that cell proliferation and peroxisome proliferation occur via different mechanisms. After 3 weeks, dietary glycine reduced basal rates of cell proliferation by about 50% and completely prevented the sustained 5-fold increase in cell proliferation caused by feeding WY-14,643. Moreover, the 3-fold increase in TNF alpha mRNA caused by WY-14,643 was blocked completely by the glycine-enriched diet. Similarly, immunohistochemical staining for TNF alpha was increased 6-fold by WY-14,643, an increase which was prevented by dietary glycine. However, the 6-fold increase in acyl CoA oxidase activity was unaffected by glycine under similar conditions demonstrating that a diet containing 5% glycine prevents increased hepatocyte proliferation caused by a potent peroxisome proliferator without affecting induction of peroxisomes. These data demonstrate that a glycine-enriched diet prevents stimulated cell proliferation most likely by inhibiting TNF alpha production and raise the possibility that dietary glycine will be effective in preventing cancer caused by nongenotoxic carcinogens such as WY-14,643.

CYP4A1 gene transfection studies and the peroxisome proliferator-activated receptor: development of a high-throughput assay to detect peroxisome proliferators

An in vitro reporter gene assay has been established to examine cytochrome P4504A1 (CYP4A1) induction. A response element from the upstream region of the rat CYP4A1 gene containing a peroxisome proliferator response element (PPRE) has been linked to the chloramphenicol acetyl-transferase (CAT) gene in a reporter vector (1). This CYP4A1 reporter construct has been co-transfected into human HepG2 cells in the presence and absence of expression vectors encoding the transcription factors PPAR alpha and RXR alpha. The assay employs calcium phosphate-DNA co-precipitate mediated transfection. Reporter gene products have been quantitated using chemiluminescent based assays. We have shown that, in the presence of PPAR alpha, the above CYP4A1 construct is transcriptionally activated by a range of structurally different peroxisome proliferators including Wy-14,643, ciprofibrate, clofibric acid and nafenopin. Our future efforts will focus on the establishment of a high-throughput assay for the detection of peroxisome proliferators. Such an assay would provide an invaluable in vitro test for the screening of developmental drug candidates prior to in vivo studies.

Peroxisome proliferators increase the formation of BPDE-DNA adducts in isolated rat hepatocytes

Peroxisome proliferators are known to modulate the activity of xenobiotic-metabolising enzymes, including glutathione S-transferase (GST) and cytochrome P-450 (CYP). In this study the effect of peroxisome proliferators silvex and di(2-ethylhexyl)phthalate (DEHP) on the formation of (+)-anti-benzo(a)pyrene -7,8-dihydrodiol-9,10-epoxide (BPDE)-DNA adducts from a proximate mutagen and carcinogen (-)-transbenzo(a)pyrene-7,8-dihydrodiol (BPDD) has been investigated. Rat CYP1A1 metabolises BPDD to mutagenic BPDE, which may form DNA adducts or, alternatively, be detoxified by hydrolysis or glutathione conjugation. In this experiment the formation of BPDE-DNA adducts was significantly increased in hepatocytes isolated from all silvex treated rats and two out of four DEHP treated rats (14 day treatment). The activity of CYP1A1 was increased whereas GST was reduced by the peroxisome proliferator silvex. These changes were more significant than those induced by DEHP. We have hypothesised that the formation of BPDE-DNA adducts was primarily due to the increased BPDD activation to BPDE versus reduced detoxication of BPDE. Other hepatic changes induced by the peroxisome proliferators, e.g. peroxisome proliferation per se and increased mitotic activity of the liver could have an effect on the outcome of BPDD exposure.

Changes in drug-metabolizing enzymes of rats in ciprofibrate-induced hepatic nodules

1. Premalignant rat liver nodules produced in the resistant hepatocyte model, by exposure to carcinogenic chemicals (diethyl nitrosamine and 2-acetamidofluorene), and partial hepatectomy, exhibit decreased xenobiotic hydroxylase activities and increased conjugase activities, which are considered responsible for increased resistance to xenobiotic toxicity. 2. However, premalignant rat liver nodules generated by feeding the hypolipidaemic, peroxisomal proliferating drug, ciprofibrate, in a hypolipidaemic model, exhibit decreased hydroxylase activities but decreased conjugase activities also. 3. It is considered that reactive oxygen species (ROS) are generated in both the resistant hepatocyte model and in the hypolipidaemic model, resulting in lipid peroxidation, loss of haem, cytochromes and hydroxylase activities. 4. However, whereas there is a rebounding compensation of conjugase enzymes in the resistant hepatocyte model, this does not occur with the hypolipidaemic model, as peroxidation is probably persistent and the conjugases are continuously destroyed.

Antioxidant-mediated attenuation of the induction of cytochrome P450BM-3(CYP102) by ibuprofen in Bacillus megaterium ATCC 14581

Bacillus megaterium contains a soluble cytochrome P450 termed BM-3, which is highly inducible by barbiturates, peroxisome proliferators, and nonsteroidal antiinflammatory drugs. In rats and mice, the chronic administration of peroxisome proliferators induces a sustained oxidative stress in hepatic tissue and may be responsible for the nongenotoxic carcinogenesis observed with prolonged treatment. Here it is shown that ibuprofen induces a variety of enzymes associated with the oxidative stress response in Bacillus, including catalase, glucose-6-phosphate-dehydrogenase, and aldehyde reductase in a dose-related manner. Furthermore, evidence is presented to show that the expression of cytochrome P450 in Bacillus is associated with a marked depletion in cellular glutathione levels and that it renders these cells considerably more sensitive to oxidant insult. Finally, this work reports that a variety of structurally diverse antioxidants such as ascorbic acid, reduced glutathione, alpha-tocopherol acetate and the artificial antioxidant, butylated hydroxyanisole, all dramatically attenuate the expression of the cytochrome P450BM-3 gene and its repressor, Bm3R1, following ibuprofen treatment. These observations provide the first evidence that the expression of cytochrome P450 genes can lead to increased oxidant sensitivity but can be strongly modulated by dietary and artificial antioxidants, as well as antioxidant enzymes. The important implications of this phenomenon are also discussed.

Mitogenic effect of retinoid X receptor agonists in rat liver

(E)-2-[2-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl-2-naphthyl) propen-1-yl]-4-thiophenecarboxylic acid (AGN 191701) and other retinoid X receptor (RXR)-selective agonists were observed to cause hepatomegaly in rats. The purpose of the present study was to understand the biochemical basis of RXR agonist-induced hepatomegaly. Male Fischer rats were implanted s.c. with osmotic pumps containing 5-bromo-2'-deoxyuridine (BrdU) and treated by gavage with 0,60, or 180 mumol/kg/day of AGN 191701 for 3 days. AGN 191701 caused dose-dependent hepatomegaly in the absence of hepatic necrosis and necrosis and increased hepatocyte BrdU labeling index (LI). To determine if AGN 191701-induced hepatic hyperplasia was sustained, rats were treated by gavage with 60 mumol/kg of AGN 191701 for up to 7 days and exposed to BrdU via osmotic pump on days 1-3 or on days 6-8. Hepatocyte L1 and mitotic index were increased only in rats exposed to BrdU on days 1-3, indicating that AGN 191701-induced hepatocyte proliferation was transient. The receptor specificity of this mitogenic effect was tested by co-treating rats for 2 days with various retinoids and BrdU. 2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-2-(4-carboxylph enyl)-1,3-dioxolane (SR11237), an RXR-selective agonist, and (E)-5-[2-(5,6,7,8-tetrahydro-3,5,5,8-pentamethyl-2-naphthyl)propen -1-yl]-2-thiophenecarboxylic acid (AGN 191659), a retinoic acid receptor (RAR)/RXR pan-agonist, both increased hepatocyte LI. Two RAR-selective agonist, all-trans-retinoic acid and (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)propen -1-yl] benzoic acid (TTNPB), did not affect hepatocyte LI. To determine if RXR agonists have biochemical effects in common with a peroxisome proliferator, various endpoints were measured 24 hr after two daily treatments with AGN 191701, SR11237, or clofibrate. While all three compounds induced hepatic acyl CoA oxidase activity, only clofibrate increased hepatic carnitine acyl transferase activity and lowered serum triglycerides. Taken together, these data show that RXR-selective agonists but not RAR-selective agonists cause hepatomegaly accompanied by hepatocyte mitogenesis in rats. The fact that RXR agonist have some biological effects distinct from RAR agonists and clofibrate suggests that RXR-selective agonists may have unique therapeutic applications.

Fish oil-feeding prevents perfluorooctanoic acid-induced fatty liver in mice

The effects of perfluorooctanoic acid (PFOA) on the levels of lipids in liver and serum were compared between mice fed a diet supplemented with soy bean oil (SO), perilla oil (PO), or fish oil (FO) for 4 weeks. Hepatic content of triglyceride (TG) was significantly lower in the mice fed the FO diet than that in the mice fed either the SO or the PO diet. The treatment with PFOA caused a marked accumulation of TG in the livers of SO-fed and PO-fed mice (seven- and twofold over their respective controls), whereas a level of TG remained low in the mice fed the FO diet. Incorporation in vivo of [3H]glycerol revealed that FO-feeding reduced synthesis of TG in the liver. The administration of PFOA increased the incorporation of [3H]glycerol into hepatic phospholipid (PL) regardless of the dietary oil, while synthesis of hepatic TG from [3H]glycerol was not altered by the treatment with PFOA. Serum level of TG was reduced by the administration of PFOA to the mice fed either the SO diet or the PO diet, while no change in the level was observed in the mice fed the FO diet. These results suggest that the accumulation of TG in the livers of PFOA-treated mice is due to the inhibition of the secretion of TG into circulation. PFOA-induced hepatic accumulation of TG is prevented by the feeding of the FO diet which inhibits TG formation. Among three dietary oils, FO-feeding alone prevented the PFOA-caused accumulation of TG in the liver. The importance of docosahexaenoic acid (22:6(n - 3)) is discussed in relation to the prevention of fatty liver induced by chemicals.

Chemiluminescence and antioxidant levels during peroxisome proliferation by fenofibrate

Fenofibrate, the hypolipidemic drug and peroxisome proliferator, was given to mice (0.23% w/w in the diet) during 1-3 weeks and H2O2 and TBARS steady state concentrations, liver chemiluminescence and antioxidant levels were measured. Administration of fenofibrate during 2 weeks induced an increase of 89% in H2O2 steady state concentration. Spontaneous chemiluminescence was decreased by 57% during fenofibrate treatment, while no significant effect was observed on TBARS concentration. Hydroperoxide-initiated chemiluminescence was decreased by 56% after 15 days of fenofibrate treatment, probably due to an increase in endogenous antioxidant levels. Total and oxidized glutathione increased gradually after fenofibrate administration, obtaining maximal increases of 67% and 58% respectively, after 22 days of treatment. An increase of 55% was found in ubiquinol levels in treated mice, as compared with the controls. alpha-tocopherol content was decreased by 51% in the liver of fenofibrate-treated mice. According to our findings, the high rate of H2O2 production associated with peroxisome proliferation, would not lead to an increase in lipid peroxidation. This can be explained by the presence of high levels of ubiquinols, which act as an antioxidant. The increased production of H2O2, would lead to DNA damage directly, and not through lipid peroxidation processes.

Photoaffinity labeling of peroxisome proliferator binding proteins in rat hepatocytes; dehydroepiandrosterone sulfate- and bezafibrate-binding proteins

To detect the cellular sites which directly interact with peroxisome proliferators (PPs) and mediate their inducing effect on peroxisomal enzymes in rat hepatocytes, two kinds of radiolabeled ligands, AD12 (7alpha-N-(4-azido-2-hydroxy-5-iodo[125I]benzyl)-aminomethyl-5-and rostene-3beta-ol-17-one-O-3-sulfate) and BZ5 (2-[p-[2-(4'-azido-3',5'-diiodo[125I]benzamido-2'-hydroxy)ethyl]phenoxy] -2-methylpropionic acid), were developed for photoaffinity labeling. These compounds were derivatives of dehydroepiandrosterone sulfate (DHEAS) and bezafibrate, respectively, with an azido group as the photoreactive functional group. Upon UV-irradiation following incubation with rat liver cytosol and nuclei, both the ligands effectively radiolabeled several proteins analyzed by SDS-polyacrylamide gel electrophoresis/radioluminography. When [125I]AD12 was used at a concentration of 0.2 microM, two cytosolic proteins with molecular masses of 55 and 28 kDa and a nuclear protein of 40 kDa were specifically labeled, as coincubation with a 1000-fold excess of DHEAS inhibited labeling. Photoaffinity labeling of the cytosolic 28-kDa protein was also affected by Wy-14,643, but not by unsulfated dehydroepiandrosterone or androsterone sulfate, consistent with our previous findings obtained in competitive binding studies of [3H]DHEAS-binding detected in rat liver cytosol (Yamada et al. (1994) Biochim. Biophys. Acta 1224, 139-146). On the other hand, [125I]BZ5 specifically labeled a cytosolic protein of 31 kDa, which was inhibited by coincubation with bezafibrate, clofibric acid and Wy-14,643, but not with DHEAS. Thus, [125I]AD12 and [125I]BZ5 labeled several proteins which recognized DHEAS and bezafibrate, respectively, in rat liver cytosol and nuclei, providing a useful means to investigate PP-binding proteins.